`Notmuch-wash-region-to-button' is the function that creates hidden
regions with buttons for signatures, citations and original messages.
Before the change, it did not work correctly if the to-be-hidden
region started at the beginning of a message: the visibility toggle
button was hidden as well. The patch fixes this. There are two parts
in the fix:
* Use `insert-before-markers' instead of `insert' for creating the
button, so that it does not get added to the hidden overlay.
* Stop using PREFIX argument for adding a newline before the button.
The newline should not be added before a button at the beginning of
buffer.
The corresponding test is fixed now.
The test is currently broken and will be fixed by a subsequent patch.
The patch adds a new file for tests of Emacs notmuch-show view.
Based on patch by David Edmondson [1].
[1] id:"1327562380-12894-4-git-send-email-dme@dme.org"
